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Matt, I thought I'd be OK wearing one of those Hi Vis jackets....no? Actually a renovation Yard, like Steptoes here in Melbourne, might have what I want.
Lee, A slipclutch is in the photo below. The central part is bolted to the shaft and the slipping part is controled by the bolts and springs which press a teflon lined plate onto the bigger outer part with the teeth on it. There is a point when there is enough friction applied, via the springs and bolts, for the worm with motor attached to rotate the gear and scope at sideral rate. When you want to move the scope by hand that is possible too while the motor is still engaged....sounds harder than it is. A Tangent arm has a couple of parts, one attached to the telescope and the other bolted to the mount. The mount part needs to have a lock/unlock clamp of some description so you can move the scope in Declination and lock it in place. The design I used on the Gaunt has a threaded rod that has limited travel of 20º either side of a central position.
The Tangent is an old way of telscope control when gears were expensive and hard to make with accuracy. I still might just add a worm and gear on the Dec axis instead as it is possible to turn it into a GoTo mount with the right electrics. My idea is Argo Navis as the digital setting circles (DSC's) so GoTo isn't really needed, just tracking with a bit of slow slewing to centre objects.
Advantages of slipclutch...no noisy motors, other than that I dunno.
Advantages of Tangent....no motors at all, cheap to build, more threaded brass rod and gears visible and I love the control rods hanging off the end of a scope, in this case an 8" f10 R50 Refractor coming out from Istar next year.
I could have just gone out and bought an EQ8 or something like that but I'd rather build my own....
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